packfile.c: fix a typo in `each_file_in_pack_dir_fn()`'s declaration

packfile.c: fix a typo in `each_file_in_pack_dir_fn()`'s declaration

One parameter is called `file_pach`. On the face of it, this looks as if
it was supposed to talk about a `path` instead of a `pach`.

However, looking at the way this callback is called, it gets fed the
`d_name` from a directory entry, which provides just the file name, not
the full path. Therefore, let's fix this by calling the parameter
`file_name` instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>